1. Field of the Invention
The invention described herein, one of a class of collector systems called a solar thermal collector system, in which radiation from the sun is converted into heat at an absorbing surface, from which this heat is collected and utilized. Three particular important aspects of the invention are approximately ideal concentration, selective absorption, and the use of a heat pipe system for transfer of the solar energy collected as heat directly into a heat storage system. In combination with a high vacuum, utilization of these three features allow the development of an integrated solar thermal collector system which can operate inexpensively and efficiently at temperatures approaching 300.degree. C. without tracking the sun, that is, as a fixed solar collector system.
2. Description of the Prior Art
Selective Absorption: A solar selective absorber for this purpose is defined as an absorber which has high absorptance for solar radiation and low absorptance, and thus low emittance, for infrared radiation of wavelengths emitted at the operating temperature of the solar collector. Solar selective absorbers with optical properties similar to that required here are discussed, for example, by A. Meinel and M. Meinel, Applied Solar Energy (Addison-Wesley Publ. Co., Reading, Mass., 1976), Chap. 9. The optical properties of the selective absorber are extremely important in obtaining high efficiency for this type of collector.
Ideal Concentration: Ideal concentration provides maximum concentration for a given angle of acceptance of solar radiation. The kind of ideal concentration considered here is cylindrical concentration onto a circular absorbing tube. The concentrating mirror surface runs the length of the absorbing tube and forms a trough. The concentration ratio is defined as the ratio of the width of the concentrating trough at the top to the circumference of the absorbing tube.
A number of patents are concerned with ideal concentration. R. Winston, U.S. Pat. No. 4,002,499, is concerned with mirror reflector shapes for cyclindrical or trough-like ideal concentration onto circular and oval cross section tubes and onto a vertical fin. A. Rabi and R. Winston, U.S. Pat. No. 4,130,107, also confine their discussion almost entirely to cylindrical concentrating mirror surfaces. In this case, they discuss a modification of Winston's patent which reduces the amount of grazing radiation on the absorbing surface, for radiation within certain pre-defined acceptance angles. W. Wyatt, U.S. Pat. No. 4,129,115, concerns himself with approximately ideal concentration suitable for a flat strip absorber, following an earlier patent of R. Winston, U.S. Pat. No. 3,923,381. He utilizes the desirable compromise of truncating the ideal concentration to reduce concentrating mirror area and thus cost. R. Lambert, U.S. Pat. No. 4,246,891, modifies the ideal concentration of Winston to be suitable for a circular cross section absorbing tube with concentric transparent vacuum enclosure.
With circular cross section absorbing tube, ideal concentration leads to a cusp in the concentrating trough which is in contact with the bottom of the absorbing tube. In order to avoid heat conduction losses from the absorbing tube to the concentrating trough, a gap is introduced between the absorbing tube and the cusp. This leads to a loss of solar radiation, called gap loss. R. Winston and W. McIntire, separately, discuss a method of putting grooves below the absorbing tube to reduce gap loss, at the expense of reduced concentration. These discussions are in Proceedings of the 1980 Annual Meeting of the American Section of the International Solar Energy Society and the journal Solar Energy, Vol. 25, pp. 215-220 (1980). Grooves are less significant for the invention disclosed here, as discussed in J. Garrison, U.S. Pat. No. 4,423,718.
A solar collector system using ideal or approximately ideal concentration with a sufficiently large acceptance angle can be oriented to receive solar radiation at all times of the year, during the daily desired hours of collection, without tracking the sun or in any way adjusting the orientation of the collector. This is an important method of greatly reducing the cost and increasing the reliability of a collector system without appreciably reducing the collector system performance.
Alternatively, the acceptance angle can be reduced to yield higher concentration, and the tilt of the collector can be adjusted a few times over the year to face the sun in each season. A rigidly fixed collector, using a selective absorber with superior optical properties, is preferred.
One patent where a collector design is indicated using approximately ideal concentration is that of A. Livermore, U.S. Pat. No. 4,134,392. He has discovered a cartesian curve describing his mirror concentrating surface which approximates the ideal concentration of Winston. He describes a solar collector involving an array of concentrating troughs with this cartesian curve shape, and with transparent tubes carrying an absorptive fluid placed in the bottom of these troughs to collect the solar energy as heat. Another case is that of D. Hervey, U.S. Pat. No. 3,321,012, who precedes Winston in the use of almost ideal concentration onto an elongated fin-like absorbing tube for the special case of a 180.degree. acceptance angle. The early work of F. Trombe, in U.S. Pat. No. 3,310,102 concerning infrared radiators, includes two reflector shapes which consist of involutes joining smoothly to plane reflectors. These reflector shapes are very similar to Winston, above. Another early reflector of this type is the Trombe-Meinel cusp, discussed in Meinel and Meinel, above, p. 204. It involves ideal concentration onto a circular cross section tube for the special case of an acceptance angle of 180.degree. .
Vacuum Collectors Without Ideal Concentration: Numerous collector designs use glass vacuum envelopes to insulate the absorbing surface, but do not use ideal concentration or approximately ideal concentration. A few examples include Abbot, U.S. Pat. No. 1,801,710; Abbot, U.S. Pat. No. 1,855,815; Emmet, U.S. Pat. No. 1,880,938; Abbot, U.S. Pat. No. 2,133,649; Godel, et al, U.S. Pat. No. 3,227,153; Mather, U.S. Pat. No. 4,002,160; Pei, U.S. Pat. No. 4,084,576; Doughty and Goodwin, U.S. Pat. No. 4,142,511; and Sims, U.S. Pat. No. 4,308,857.
Heat Pipes for Thermal Control: B. S. Larkin, in Heat Transfer 1982, Vol. 6, Proceedings of the Seventh International Heat Transfer Conference, Muenchen, Federal Republic of Germany, treats the case where a heat pipe can be designed to transfer no heat above a given temperature by adjusting the quantity of working fluid in the heat pipe. This kind of heat pipe can limit the peak temperature of a storage reservoir receiving heat from this heat pipe.
Many sources treat the subject of variable conductance heat pipes where an inert gas is inserted in a heat pipe with the working fluid. A variable conductance heat pipe can be used to regulate the temperature of a heat source, such as a storage system. See for example, P. Dunn and D. Reay, Heat Pipes (Pergamon Press, New York, 1976) pp. 199-231.
Heat Storage Systems: Storage systems are discussed in J. Duffie and W. Beckman, Solar Energy Thermal Processes (John Wiley and Sons, New York, N.Y., 1974) Chap. 9 and D. Rapp, i Solar Energy (Prentice-Hall, Englewood Cliffs, N.J., 1981) Chap. 13, and their references, for example.
Vacuum Collectors Using Heat Pipes: J. Ribot and R. McConnell in Journal of Solar Energy Engineering, Vol. 105, pp. 440-445 (November 1983) (See also U.S. Pat. No. 4,474,170) describe an evacuated solar thermal energy collector using a selective absorber coating on a glass walled heat pipe in vacuum for collection and transfer of the collected heat to an external manifolding. A. Slaats, in U.S. Pat. No. 4,335,709, describes an evacuated collector tube using an absorbing metal heat pipe. The collector tube design is somewhat similar to that of Ribot and McConnell. J. de Grijs and H. Bloem, in U.S. Pat. No. 4,356,811, G. W. Knowles, U.S. Pat. No. 4,119,085, H. van der Aa, U.S. Pat. No. 4,416,261, F. Sabet, U.S. Pat. No. 4,311,131, and W. Kroontje and G. Kuus, U.S. Pat. No. 4,455,998 describe an evacuated solar collector tube using a metal heat pipe with solar heat absorber plate. None of the above collecters using heat pipes use ideal or approximately ideal concentration in their collector tubes. Ortabasi and Buehl, in Proceedings of the 1977 Annual Meeting of the American Section of the International Solar Energy Society and later in the journal Solar Energy, Vol. 24, pp.477-489 (1980), have introduced a glass evacuated collector tube with internal, approximately ideal reflector, which uses a metal heat pipe to remove the solar heat from the collector tube.
Collector Systems with Passive Heat Transfer Directly from Collectors to Storage Medium: One of the collector systems similar in appearance, but rather different from, the invention presented here is the commercially produced Solahart flat plate system (Solahart of California Corporation). This system has the storage tank at the upper end and above the collector, and uses thermal convection to cause closed circulation of heated fluid from the collector to the storage tank, and the return of cooler fluid into the lower end of the collector. The system of K. T. Feldman, U.S. Pat. No. 4,217,882, uses an absorbing heat pipe to receive solar radiation focussed or concentrated by a reflector, and transmits heat obtained by this radiation to a storage reservoir. Other systems include those of R. French, U.S. Pat. No. 4,240,405; B. Hunter, U.S. Pat. No. 4,505,261; K. K. Sharp and E. Okamoto, Japanese Pat. No. 55,033; and D. Matsushita and M. Masaharu, Japanese Pat. No. 195,744.
Vacuum Collectors Using Approximately Ideal Concentration: The collector designs of L. Dorbeck, U.S. Pat. No. 4,198,955, use the features of vacuum, approximately ideal concentration, and selective absorption, which are required of a solar collector which is to be highly efficient at elevated temperatures, while still being low in cost. The tubular collectors described by Dorbeck are similar to those proposed by J. Garrison at about the same time in Proceedings of the 1977 Annual Meeting of the American Section of the International Solar Energy Society, and more recently reported in the journal Solar Energy, Vol. 23, pp. 93-102, 103-109 (1979). The solar collector panel design of J. Garrison, U.S. Pat. No. 4,423,718 is somewhat similar to the panel of Dorbeck. This panel of Garrison carries the concept of functional integration an important design improvement beyond Dorbeck, integrating the manifolding inside the panel. The choice of an all glass vacuum envelope for the Garrison panel is also important by simplifying fabrication and reducing materials costs. The designs of Garrison, Dorbeck, and Ortabasi and Buehl comprise those collectors closest in concept to the design of the solar collectors in the solar thermal collector system of this application.
Other collectors using approximately ideal concentration, vacuum and selective absorption include: General Electric Company collectors TC 100 and TC 120 (following Lambert, above); and Energy Design Corporation (Memphis, Tenn.) collectors XE-300 (similar to the Argonne National Laboratory design described without vacuum by A. Rabl, et al in Solar Energy, Vol. 25, pp. 335-351 (1980)) and the more recent HP-250.
Prior Art Deficiencies: As discussed, numerous designs for solar thermal collector systems have appeared over the years in patents and in the published literature. All of these designs apparently function properly, though some are more highly preferred by virtue of lower cost, greater durability, and/or higher efficiency. A major problem has been the cost of this energy relative to the cost of energy from other sources. Another problem has been the inability of most of these collectors to operate at sufficiently elevated temperatures, or under conditions of low ambient temperatures or low solar radiation. Practically all of these designs have not reached the market place, and those that have, generally have been unable to compete in price with other sources of energy.
Further, the prior art solar collectors are inefficient and/or high cost relative to the solar thermal collector system design presented here, either by lacking certain design features which are important for efficient solar energy collection, or by lacking design or production features needed to achieve low cost, or for both of these reasons. The important features required to obtain high efficiency and low cost are selective absorption, vacuum, approximately ideal concentration, the use of low cost materials and processes for fabrication, and functional integrity, not just of the collector, but of the combined collector, storage and energy utilization system. Functional integrity here means using one structural element to serve more than one function, thus simplifying design and generally reducing cost.
In the prior art, the system of K. T. Feldman uses a metal heat pipe to transfer the solar heat to a storage reservoir, in a manner similar to that of the invention of this application. This solar heat is obtained by an adjustable concentrator concentrating solar radiation on this heat pipe. Although there can be vacuum, approximately ideal concentration, and selective absorption, this collector system is very different from the collector of this invention, primarily because it is not functionally integrated and also uses different materials. This is also true for the collector of Sharp and Eiki.
The collector tube of Ortabasi and Buehl uses a metal heat pipe to transfer the collected solar energy outside the collector tube, where this energy is removed by passing a heat transfer fluid past this outer end of the heat pipe. This collector tube uses a glass-to-metal seal, which is more expensive. Arrays of these tubes require manifolding and piping with insulation to transfer the collected heat to storage, also at added cost and lower efficiency. Further, the approximately ideal concentration used in this collector tube is separate from the vacuum envelope. Metal concentrators formed this way have lower reflectivity and higher cost than the silvered glass concentrator used with the tubes of Garrison and Dorbeck, and the panel of Garrison, where the vacuum envelope function and the concentration function are integrated.
The Collector tubes of Garrison and Dorbeck have integrated the concentration function and the vacuum envelope function, but require insulated external manifolding and piping to collect the heat from an array of these tubes and transfer this heat to storage. In addition, these tubes generally require either glass-to-metal seals, graded seals, or more expensive low expansion glass, at higher cost.
The panel of Garrison integrates the manifolding function, the concentration function, and the vacuum envelope function, but requires a pump to pass fluid through an array of these panels. Further, these panels require expensive glass-to-metal seals or graded seals, though many fewer for an array than the tubes of Dorbeck, Garrison, or Ortabasi and Buehl.
The above collector tubes of Dorbeck, Garrison, and Ortabasi and Buehl, and the panels of Dorbeck and of Garrison utilize vacuum, approximately ideal concentration, selective absorption and varying degrees of functional integrity. They also attempt to use low cost materials and processes to a varying degree. However, none of them take the important added step of carrying the functional integration and the design integration to a more complete solar thermal collector system. This added step yields very significant savings in the cost, ease of fabrication, and reliability of operation of the invention of this application, and it also contributes to a higher energy collection efficiency of this invention.
Although the tubes and panels of Garrison, Dorbeck, and Ortabasi and Buehl, above, are preferred over earlier collectors, there are a number of decisions to be made and difficulties associated with these decisions which have not been resolved previously. The main decisions are: (1) whether to use glass or metal for the absorber tube, and which glass or metal, (2) whether to use a heat pipe or fluid flow to remove the solar heat, (3) with a heat pipe, what fluid and geometry to use, and with fluid flow, what pipe flow geometry and heat transfer fluid should be used, and (4) whether to use more expensive, low thermal expansion glass or less expensive, high thermal expansion glass.
Some of the difficulties associated with the above decisions are as follows: The decision to use a tube collector with a metal absorbing tube leads to the requirement of glass-to-metal seals which are expensive and make tube fabrication more difficult. Similarly, a glass absorbing tube must apparently be borosilicate glass or other, more expensive, low expansion glass to avoid cracking of the glass when cool fluid is pumped through hot glass or vice versa at start up. The vacuum envelope could be high thermal expansion soda lime glass to reduce the cost of the vacuum tube. This then requires an expensive graded seal between the absorbing tube glass and the vacuum envelope glass. If straight flow through the tube is desired, then two glass-to-metal seals or graded seals and a bellows or similar device of added cost are required to accommodate for the thermal expansion of the absorbing tube. If a counter flow design is used, either the ideal concentration is compromised, or the ease of connecting the manifold is compromised. Collector tubes have been connected together in an array using an external manifolding, which has higher cost, higher losses, and lower energy collection than the panel of Garrison. Series fluid flow through an array either requires higher pumping pressure, or, if lower pumping pressure is used, this leads to higher radiation losses. Parallel flow requires careful design of the flow network to allow flushing out the bubbles in the network, and can have greater difficulty with drain-down, which is commonly used to avoid problems with fluid freezing. Drain-down can be avoided by using a fluid other than water, which does not freeze at the lower ambient temperatures, but these fluids are usually considerably more expensive and poorer heat transfer fluids than water. If drain-down is not practiced, then, if the pump shuts down during the daytime, the high stagnation temperatures achieved by the system can lead to excessively high pressures, perhaps breakdown of the fluid, and perhaps a fire. The few suitable fluids known to us which appear not to have these problems are quite expensive. Most heat transfer fluids other than water are polluting, if lost from the system, and are usually flammable.
Heat pipes used with high efficiency solar collector tubes or panels having vacuum, approximately ideal concentration, and selective absorption can experience high pressures at the stagnation temperatures possible for this type of collector. This can make the use of heat pipes less desirable, unless the maximum temperature is controlled, as in the invention of this application.
This patent application is a continuation-in-part of patent application with Ser. No. 702,401 and patent application with Ser. No. 794,010. Each of these earlier applications contain different embodiments of what is basically the same invention. There is another additional embodiment, which will also be included in the present application. There should be only one patent application for the different embodiments of this invention, with one generic claim and separate claims for each of the different embodiments.